In internal combustion engines, there is a cylinder injection system designed to cause an injector to inject fuel directly into a combustion chamber. The chamber is formed by a cylinder block, a cylinder head, and a piston.
In recent years, many proposals have been made to a gasoline engine of a direct cylinder injection type to force the fuel directly into a cylinder (for example, see published Japanese Laid-Open Patent Application No. 5-71350).
In most of these gasoline engines, the following steps are provided during low load engine operation: fuel is blown into a piston cavity during engine compression strokes, the piston cavity being a concave portion that is provided on a top surface of a piston; fuel is gathered near spark plugs; an air-fuel mixture is stratified, whereby the engine is entirely operated with a lean air-fuel ratio so as to provide a considerable improvement in fuel efficiency. Meanwhile, different steps are provided during high load engine operation: fuel is injected into a cylinder during intake strokes, thereby producing a uniform air-fuel mixture in the cylinder; and the engine is operated with either a theoretical or richer air-fuel ratio. The engine is characterized in that the angle of injected fuel spray as well as the distance for atomization varies with fluctuations in the internal pressure of the cylinder due to a change in injection timing for swirl type injections. With further reference to such a variation, when the internal cylinder pressure is lower, then the spray angle is smaller, while the distance of atomization is longer. In a converse case, the spray angle is greater, but the distance of atomization is shorter. Most of the presently proposed direct cylinder injection engines with externally supplied ignition are of this type.
In addition, a high level of fuel pressure (e.g., 5 MPa or greater) is retained in order to atomize the fuel spray. Five MPa is equal to 50,000 hPa, which is some fifty times as great as atmospheric pressure because atmospheric pressure is about 1,013 hPa.
One example of a cylinder injection system engine is disclosed in published Japanese Laid-Open Patent Application No. 5-118244. The internal combustion engine disclosed therein performs fuel injection by dividing requested fuel injection quantity according to intake and compression strokes. In this engine, one fuel injection quantity for each intake stroke is determined by the requested fuel injection quantity being multiplied by a partition factor. Another fuel injection quantity for each compression stroke is determined by the above determined injection quantity being subtracted from the requested fuel injection quantity. In this way, the engine is designed to insure satisfactory combustion.
Another example is disclosed in published Japanese Laid-Open Patent Application No. 5-118245. The internal combustion engine disclosed therein performs fuel injection by dividing requested fuel injection quantity according to intake and compression strokes. In this engine, one fuel injection quantity for each intake stroke is determined by the requested fuel injection quantity being multiplied by a partition factor that is based on the internal pressure of a cylinder. Another fuel injection quantity for each compression stroke is determined by the above determined injection quantity being subtracted from the requested fuel injection quantity. The engine thereby reliably provides satisfactory combustion.
A further example is disclosed in published Japanese Laid-Open Patent Application No. 7-119507. The cylinder injection system of a spark ignition engine disclosed in '507 includes a fuel injection valve that starts fuel injection during each intake stroke in an engine-operating state in which uniform combustion is executed. In this engine, in order to provide fuel injection, a required amount of fuel is divided a number of times during at least low engine rotational speed in an engine operating state. As a result, smoke is prevented from occurring during the uniform combustion.
A still further example is disclosed in published Japanese Laid-Open Patent Application No. 9-32619. The cylinder injection system of an internal combustion engine disclosed in '619 includes a fuel injection valve for injecting fuel directly into a combustion chamber of the engine, thereby effecting lean combustion operation with an air-fuel ratio greater than a theoretical air-fuel ratio. The engine is characterized by: an exhaust passage for discharging exhaust gases from the combustion chamber; a lean NOx catalyst disposed in the exhaust passage for absorbing nitrogen oxides (NOx) in the exhaust gases during the lean combustion operation; an operating state-detecting means for detecting how the engine is running; and a fuel injection valve control means for supplying the lean NOx catalyst with additional fuel by actuating the fuel injection valve during each exhaust stroke of the engine based on the above detection result from the operating state-detecting means. As a result, materials absorbingly adhered to the lean NOx catalyst, which materials contribute to a reduction in purification capability, are eliminated securely and properly without torque fluctuations being involved.
In the compression stroke injection of conventional cylinder injection system engines, fuel injection timing from an injector must be retarded as much as possible in order to ensure that fuel is trapped in a piston cavity. The cavity has a concave shape, and is formed on the upper surface of the piston.
In order to ensure that the spark plugs ignite the air-fuel mixture, the fuel injected from the injector must be reliably atomized at the very moment of such ignition ("a combustion setup period"). In order to insure the combustion setup period, the injection timing of fuel from the injector must be advanced as much as possible.
In particular, the injection period and the combustion setup period with reference to a crank angle of the crankshaft increases with an increase in engine rotational speed and load (within a range of the compression stroke injection). Thus, it is difficult to satisfy the aforesaid two requirements at a single time, and consequently there has been a continuing desire to overcome such difficulty.
Further, in the compression stroke injection of the cylinder injection system engine, the injection period with reference to the crank angle is made longer with elevated engine rotational speed and load. This causes an inconvenience in that a sufficient period of time required for fuel atomization cannot be guaranteed, which is a disadvantage in practical use.